Field of the Invention
The present invention relates to magnetic resonance imaging (MRI) systems and in particular to arrangements for reducing the vibrations of gradient coil assemblies.
Description of the Prior Art
As is well known in the art, a typical MRI system comprises a superconducting main magnet which generates a strong, constant background field with a homogeneous region which contains an imaging region. Gradient coils are provided to generate oscillating orthogonal magnetic fields in the imaging region, which cause resonance of atomic spins of atoms within an imaging target, typically a human patient.
The interaction of the oscillating magnetic fields generated by the gradient coils with the constant background field causes strong mechanical vibrations of the gradient coil assembly, in turn causing unpleasant and disturbing noise for a patient.
FIG. 1 shows a radial cross-section through a typical magnet system for use in an MRI imaging system. A cylindrical magnet 10, typically comprising superconducting coils mounted on a former or other mechanical support structure, is positioned within a cryostat, comprising a cryogen vessel 12 containing a quantity of liquid cryogen 15, for example helium, which holds the superconducting magnet at a temperature below its transition temperature. The magnet is essentially rotationally symmetrical about axis A-A. The term “axial” is used in the present document to indicate a direction parallel to axis A-A, while the term “radial” means a direction perpendicular to axis A-A, in a plane which passes through the axis A-A. The direction z is the direction along the axis A-A; the direction x is the vertical radial direction and the direction y is the horizontal radial direction.
The cryogen vessel 12 is itself cylindrical, having an outer cylindrical wall 12a, an inner cylindrical bore tube 12b, and substantially planar annular end caps (not visible in FIG. 1). A vacuum vessel 14 surrounds the cryogen vessel. It also is itself cylindrical, having an outer cylindrical wall 14a, an inner cylindrical bore tube 14b, and substantially planar annular end caps (not visible in FIG. 1). A hard vacuum is provided in the volume between the vacuum vessel 14 and the cryogen vessel 12, providing effective thermal insulation. A thermal radiation shield 16 is placed in the evacuated volume. This is typically not a fully closed vessel, but is essentially cylindrical, having an outer cylindrical wall 16a, an inner cylindrical bore tube 16b, and substantially planar annular end caps (not visible in FIG. 1). The thermal radiation shield 16 serves to intercept radiated heat from the vacuum vessel 14 before it reaches the cryogen vessel 12. The thermal radiation shield 16 is cooled, for example by an active cryogenic refrigerator 17, or by escaping cryogen vapour.
In alternative arrangements, the magnet is not housed within a cryogen vessel, but is cooled in some other way: either by a low cryogen inventory arrangement such as a cooling loop, or a ‘dry’ arrangement in which a cryogenic refrigerator is thermally linked to the magnet. In ‘dry’ configurations, heat loads on the magnet are not directly cooled by liquid cryogens but, instead, are removed via a thermal link connected to a cooling pipe or refrigerator. Such heat-loads can result, for instance, from current ramping or gradient coil operation.
The vacuum vessel bore tube 14b must be mechanically strong and vacuum tight, to withstand vacuum loading both radially and axially. Conventionally, it is made of stainless steel. The cryogen vessel bore tube 12b, if any, must be strong and capable of withstanding the pressure of cryogen gas within the cryogen vessel. Typically, this is also of stainless steel. The bore tube 16b of the thermal radiation shield 16 must be impervious to infra-red radiation. It is preferably lightweight and a good conductor of heat. It is typically made of aluminium.
The present invention may be applied in all such cases.
In order to provide an imaging capability, a set of gradient coils 20 are provided within a gradient coil assembly 22 mounted within the bore of the superconducting magnet. A gradient coil assembly usually comprises a hollow cylindrical, resin-impregnated block, containing coils which generate orthogonal oscillating magnetic field gradients in three dimensions.
During an imaging procedure, the gradient coils 20 generate rapidly oscillating magnetic fields with very fast rise-times of typically just a few milliseconds. Stray fields from the gradient coils generate eddy currents in metal parts of the cryostat, in particular in metal bore tubes 14b, 16b, 12b of vacuum vessel, thermal shield and cryogen vessel, and also in the structure of the magnet 10. The eddy currents produced in the material of the vacuum vessel 14 will help to shield the thermal radiation shield 16 and cryogenically cooled components such as cryogen vessel bore tube 12b, magnet coils and magnet former 10 from stray fields from the gradient coils 20. However, because of the constant background magnetic field produced by the magnet, those eddy currents produce Lorentz forces, acting radially and axially and resulting in mechanical vibrations in the bore tube of the vacuum vessel. Further mechanical vibrations result from mechanical vibration of the gradient coil assembly itself, caused by Lorenz forces acting on the conductors of the gradient coil assembly 22 which carry significant rapidly alternating currents. Mechanical vibration of the gradient coil assembly due to Lorenz forces acting on the conductors within the gradient coil assembly also causes noise by direct vibration of air within the bore.
These mechanical vibrations, in the constant background magnetic field of the magnet 10, will in turn induce secondary eddy currents in conductive materials, such as the bore tube 16b of the thermal radiation shield, or the bore tube 12b of a cryogen vessel. The secondary eddy currents will of course generate magnetic fields, known as secondary magnetic fields. These can interfere with imaging, and also produce mechanical vibrations and secondary stray fields. The secondary stray fields also induce tertiary eddy currents in nearby conductive surfaces. These tertiary eddy currents will, in turn, generate tertiary magnetic fields, and so on. By this mechanism, the gradient coil is able to deposit significant heat energy in the magnet and/or surrounding cryogen vessel. Steps should therefore be taken to limit the vibration of the gradient coil itself. It is also desirable to reduce the amplitude of vibration of the gradient coil so that the noise and vibration experienced by the patient during imaging is reduced.
The bore tube 16b of the thermal radiation shield 16 is preferably thermally and electrically conductive to provide electromagnetic shielding of the magnet from the gradient coils.
A particular difficulty arises when, as is typical, the frequency of oscillation of the gradient magnetic fields is close to the resonant frequency of the bore tubes. It is known that a number of concentric bore tubes of similar diameters, such as the bore tubes of the vacuum vessel, thermal radiation shield and cryogen vessel of a typical MRI system, have similar effective resonant frequencies when made from common engineering materials as previously described.
The mechanical vibrations will be particularly strong when a resonant vibration frequency of a bore tube corresponds to the frequency of oscillation of the stray field. If the resonant frequencies of the vacuum vessel bore tube, thermal shield bore tube, cryogen vessel bore tube if any, and magnet components are close together, as is the case in current magnets, the bore tubes behave as a chain of closely coupled oscillators, and resonance bands will occur.
The oscillations may also interfere with the imaging process, causing detriment to the resulting images.
The resulting oscillations cause acoustic noise and vibration which is most unpleasant for a patient in the bore, as well as interfering with imaging and causing heating of cooled components such as the thermal radiation shield and cryogen vessel, if any.
The eddy currents induced in the cryogenically cooled components of the magnet constitute an ohmic heat load on the cryogenic cooling system, leading to an increased consumption of liquid cryogen where used, or an increased heat load on the cryogenic refrigerator. In dry magnets—those which are not cooled by a liquid cryogen—the increased heat load can result in a temperature rise of the coils, which can result in a quench.
Known approaches to this problem include the following. The gradient coil assembly 22 may be mounted to the vacuum vessel bore tube 14b using resilient mounts, wedges or air bags. These are intended to attenuate the mechanical oscillations of the gradient coil assembly. However, such arrangements do not completely prevent mechanical transmission of vibrations from the gradient coil to the vacuum vessel, and do very little to reduce the incidence of eddy currents in adjacent electrically conductive structures. It has been suggested to mount the gradient coil on to end frames, rather than to the vacuum vessel bore tube. However, such arrangements have required a lengthening of the system, which the present invention also seeks to avoid. Active force feedback actuators are suggested in U.S. Pat. No. 6,552,543, where actuators are placed within the vacuum vessel to oppose vibrations caused by stray fields from gradient coils. This solution is considered complex, and difficult to position the actuators between other components such as the magnet coils. Mode-compensated gradient coils have been suggested, in which primary and secondary conductors of the gradient coil assembly itself are optimised to reduce the amplitude of vibration of the gradient coil assembly. However, such optimisation makes it more difficult to achieve other important required gradient coil design objectives such as minimising the stray field.
Known approaches to similar problems have been set out in the following publications.
U.S. Pat. No. 6,552,543 B1 (Dietz et al., Siemens) discloses the use of mountings, including active mounts, between the gradient coil assembly and the cryostat.
U.S. Pat. No. 5,345,177 B2 (Sato et al, Hitachi) this discloses the use of radial-spoke gradient coil mountings incorporating soft pads.
U.S. Pat. No. 6,353,319 B1 (Dietz et al., Siemens) discloses mounting the gradient coil in the magnet bore, at points of maximum amplitude of mechanical vibrations, to disrupt resonant modes.
U.S. Pat. No. 7,053,744 B2 (Arz et al., Siemens) discloses a vacuum enclosure for the gradient coil.
U.S. Pat. No. 5,617,026 (Yoshino et. al, Hitachi) discloses the use of Piezo-transducers as a means of reducing the amplitude of gradient vibrations.
DE 10 2007 025 096 A1 (Dietz et al., Siemens) discloses a method of mode-compensation of a gradient coil.
To be effective in reducing vibration of the gradient coil assembly, the following issues must be addressed. Lorentz forces within the gradient coil should be compensated to avoid unbalanced loads in the gradient coil assembly which would tend to cause large amplitude vibrations resulting in high levels of acoustic noise and gradient coil induced heat load (GCIH). The effective flexural stiffness of the gradient coil assembly must be greatly increased, to reduce the amplitude of any vibrations, and to increase the resonant frequencies of the gradient coil assembly. The gradient coil assembly should be mechanically isolated from the cryostat to prevent direct excitation of vibrations in the vacuum vessel which generate increased noise. These issues should be addressed without causing any increase on the required diameter of the primary superconducting coils 30, or any reduction in available patient bore diameter.